arthropod vector, efforts to control the vector can be crucial in containing or halting an outbreak. This is true even for those vector-borne diseases, such as yellow fever or malaria, for which there is or may eventually be an effective vaccine. To be effective, a vaccine must have time—often several weeks—to elicit an immune response in recipients. Vector control may provide this opportunity (see Box 3-5).

BOX 3-5Vector Control in Action

Venezuelan equine encephalomyelitis was introduced into Texas in 1971. This was not a new virus but a highly pathogenic (in both equines and people) strain that had emerged in Central America in 1969. The disease advanced from Guatemala through Mexico and into Texas, a distance of more than 4,000 kilometers, in two years. The virus produced high-titer viremias in equines and was isolated from many species of mosquitoes that fed on equines and people. Most of these mosquito species previously had been considered to be pests rather than vectors of disease (Sudia et al., 1975).

The initial approach to containing the epidemic was to immunize equine populations (horses, mules, donkeys, and burros) across extensive areas of Central America and Mexico. The objective was to create an immunological barrier to prevent further spread. Fortunately, a vaccine developed by U.S. Army researchers (Berge et al., 1961) had been stockpiled, and additional doses were rapidly prepared. Although more than 4 million equines were vaccinated in Mexico in a two-year period, the virus continued to spread. There were tens of thousands of equine cases and 8,000 to 10,000 equine deaths in Mexico alone. Almost 17,000 cases (but no deaths) were reported in humans (Sudia et al., 1975).

Once it was recognized that the disease had invaded Texas, a massive campaign to eliminate the virus was initiated (Pan American Health Organization, 1972). A total of 2.25 million equines were vaccinated over an 11-state area, and a quarantine was established to prevent movement of the equines out of infected areas. Malathion and dibrom pesticides were applied over 8 million acres in Texas and Louisiana to control mosquito populations. With completion of these activities in 1972 and the onset of winter, the pathogenic strain of the virus disappeared from Texas, Mexico, and Central America. The program's cost exceeded $30 million (Sudia et al., 1975).

The virus has not reappeared, and it must be assumed that the vaccinated equine population has, after 20 years, been replaced by susceptible animals. Thus, this region is now receptive to the reintroduction of a pathogenic virus from South America or to the reemergence of a virulent strain from the Venezuelan equine encephalomyelitis viruses endemic in Central America and Florida.

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